Magnetic Behavior of Single Magnetite Grains: Insights from Micromagnetics and Micromagnetometry

Paleomagnetic signals from bulk rock samples are often affected by the magnetic properties of individual minerals and the limitations of laboratory methods. Focusing on understanding single magnetic grains enhances the reliability of these signals and refines their interpretation. Advances in micromagnetometry have significantly improved magnetic field imaging capabilities, enabling detailed analyses at the grain level. In this study, quantum scanning microscopy (QDM) and magnetic force microscopy (MFM) were employed to measure magnetic stray fields and domain states of a ~4.5 µm³ large single magnetite grain. Micromagnetic simulations using MERRILL (Conbhuí et al. 2018) were conducted to investigate the grain’s magnetic configurations under varying external fields. The aim was to understand the grain's magnetic behavior in response to these fields and to establish correlations between experimental observations and simulation results.

Four hundred simulations revealed that the magnetite grain, due to its size, symmetry and crystallographic orientation, developed multi-vortex structures with magnetization concentrated near its boundaries. The vortex cores exhibited intricate configurations and were not aligned with specific crystallographic axes. Mean magnetization varied by approximately 38% after exposing to the external field ranged from 100 to 150 mT. The average dipole moment orientations shifted by up to 40° across this field range. After exposure to an external field of 700 mT, the dipole moment orientations became highly stable.

Surface scans showed the evolution of magnetic domains under different external field strengths, particularly at scan heights below 400 nm. At 300 nm, the stray field intensity reached 5 mT, with Bz extremes localized at the grain corners. At scan heights exceeding 500 nm, the stray field patterns transitioned into dipole configurations, obscuring domain-level details. Despite variations in domain structures, dipoles exhibited consistent shapes at higher scan heights. These findings highlight the magnetite grain's response to external fields, with simulations providing valuable insights into its magnetic domain state developments. Systematic analyses with experiements and simulations will enable the differentiation of magnetically stable and unstable grains based on their shape, size, and composition, improving the assessment of magnetic grains as reliable paleomagnetic recorders.

Reference: Ó Conbhuí et al. (2018). Geochemistry, Geophysics, Geosystems, 19, 1080–1106.